It is desirable to carefully control the intravenous (IV) administration of liquids to a patient. Conventional gravity IV solution delivery via commonly-available IV administration sets is typically not sufficiently accurate for the delivery of many types of fluids and drugs. Various positive displacement pumping devices have been developed for carefully controlling such IV administration. Some types of IV pumps control flow within a standard IV administration set via peristaltic (either linear or rotary) pumping schemes directly on the tubing of a conventional IV infusion set, other types may incorporate a proprietary volumetric cassette, still others utilize a syringe-like device. However, there currently exists no IV controller(s) capable of completely safe operation within a MRI suite wherein a considerable need exists for the controlled delivery of medicinal liquids. Frequently, patients scheduled for MRI examination arrive at the MRI suite with IV solutions being administered and controlled by devices which must be disconnected as the patient is moved into the suite where high magnetic fields are present and no outside RF interference can be tolerated.
The basic characteristics of an infusion pump involve the delivery of medicinal or nutritional liquids, over time, into the venous system of a living subject. Certain physical limitations regarding the delivery rate and pressure are elemental in IV liquid-infusion control. IV fluids are pumped at pressures typically in the range of 0.2 to 10 PSI. The infusion device should include detection of over-pressure and operational limits at not more than about 20 PSI. Flow ranges typical of IV pumps are from 0.1 to 2000 ml/hr. Such specifications for conventional IV infusion apparatus are quite different from the specifications for Injector devices which are often used in radiologic settings, including MRI, for purposes of very rapid bolus injection of image enhancing contrast agents. Such devices “push” contrast agents at pressures up to 300 PSI and in very short periods of time in contrast to IV drug delivery. Contrast agents are solely for image enhancement and have no medicinal value in a patient.
The high magnetic field surrounding MRI systems can negatively affect the operation of various devices (including conventional IV control devices), especially those devices that are constructed with magnetic materials, and can seriously jeopardize a patient's safety as a result of devices utilizing magnetic materials that can be attracted at high velocity into the magnetic field of the MRI system where patient or attendant personnel are located.
Conventional devices for infusing liquids into a patient are typically small portable units often attached to an IV pole holding both the infusion device and associated liquids to be infused. Some of such devices utilize either stepper-type motors or simple DC motors which include magnetic materials for providing the mechanical power required to drive the pumping unit. Further, some form of electronic control unit receives the user's prescribed infusion rate settings and controls the pumping unit to deliver the desired quantity of liquid over time, and such control unit may emit spurious radio frequency signals as a result of poor electrical design or insufficient shielding.
With the advent of MRI procedures for the imaging of internal body structures, very special requirements must be satisfied in the design of medical devices intended to be used within the MRI environment. MRI systems exploit the physical phenomenon of nuclear magnetic resonance (NMR) by which RF stimulation of atomic nuclei within an associated magnetic field results in the emission of a small RF “spin echo” from the nucleus so stimulated. In the case of patient imaging, hydrogen nuclei bound with water are the usual targets for magnetic resonance at selected frequencies. Other molecules and compounds can also be selected for study, as in Nuclear Magnetic Spectroscopy, by choosing resonance specific magnetic field strengths and associated radio frequencies. For simplicity the typical hydrogen atom-based MRI image-acquisition process is referred to herein, but it should be recognized that the subject invention is equally useful in MRI spectrographic studies at a plurality of field strengths and frequencies.
The typical MRI system includes several components, as shown in FIG. 1. For example, the operator's console 25, 27 and various processing 37, display 29, 31 and radio frequency and magnetic gradient amplifying equipment 33, 35 are all located outside of the environment of the MRI scanning suite which must be configured to eliminate image-degrading radio frequency interference and field effects of metallic structures that can introduce field distortions and become safety hazards. The MRI scanning unit produces large magnetic and RF fields, and must be capable of receiving the extremely small RF nuclear “echoes”, and is therefore typically located within a shielded room 11. Such rooms greatly attenuate outside RF noise and may also some provide containment of the scanner's magnetic field.
However, certain devices are required to be placed in the scan room either to assist with care of the patient being imaged or for the use of attending staff. Of particular interest are those devices which must be placed in the scan room during the time of image acquisition when the patient is present and the magnetic fields are “up” and RF reception of the tiny nuclear “echoes” must be cleanly acquired. Electrically passive metallic items such as oxygen bottles or “crash carts” present safety hazards to the patient due to their potential to be strongly attracted by the scanner's magnetic field. Such items can be “pulled” into the imaging volume where the patient is located, creating potential for serious injury or death. Additionally, great effort is made during the manufacture and installation of the scanner/magnet to assure that the lines of flux within the imaging volume are highly homogenous to assure that acquired images have minimal spatial distortion. Thus, devices formed of magnetic material that are positioned within the scanner's magnetic field can introduce distortions into this homogeneous field and the resultant images. The level of hazard and the degree of field/image distortion due to magnetic materials depends upon the composition and location with respect to the imaging volume.
The hazards due to “flying” objects can be controlled to some degree by the use of non-ferrous devices such the aluminum oxygen bottle. Additionally, the gravitational weight of some devices or their rigid fixation in the scanning room maybe sufficient to overcome the force of magnetic attraction on the ferrous mass of such devices toward the imaging volume. However, such devices with some ferrous mass, though inhibited from being pulled into the magnetic field, may nevertheless introduce inhomogeneity in the magnetic field. Distortions in the homogeneity of the magnetic field within the imaging volume must be kept at such a level as to be of minimal consequence to the operator reading the resultant image or data. And, the possibility of field distortion is proportionally increased as devices with metallic materials are positioned closer to the imaging volume, with the most critical position being near the center of the imaging volume, essentially where the patient is positioned. Additionally, because of the extremely low levels of RF signals produced by the target image nuclei, great care must be taken to assure that devices with active electronic circuits do not emit spurious RF signals as forms of electronic noise. Such noise can so degrade the signal-to-noise ratio of signals received by the MRI sensor coils and receivers that image resolution is reduced or rendered completely unreadable. Active circuits must be carefully shielded to assure that their RF emissions are extremely low at the specific frequencies of the imaging process. Conversely, it is possible through careful design, to place a source of RF energy for signal transmission, therapy, or the like, within the MRI environment, but such signals must be chosen to avoid the discrete Larmor frequencies unique to the particular magnetic field strength of a given MRI scanner, and must be of such high spectral purity as to coexist with the MRI without causing any deleterious effects. The intense magnetic fields produced by the MRI scanner can cause detrimental effects on the performance of common DC and stepper motors in devices needed within the MRI scanning room, to the point of making their control difficult or causing their complete failure.
For example, injectors of image-enhancing contrast agents are commonly required to inject such contrast agent during actual imaging acquisition, and such devices include motors that contain magnetic material and that must therefore be located at a sufficient distance to reduce interactive effects with the magnet of the MRI scanner for proper operation and safety. Controllers and consoles of electronics and displays that generate spurious RF signals are therefore located outside the MRI scan room to avoid interference with the sensitive RF receivers of the RF scanner.
Accordingly, it is desirable to provide a self-contained, MRI-compatible infusion pump for the relatively long term control and delivery of the various infusion solutions and drugs routinely delivered to a patient within the MRI environment during image acquisition. Such devices must not emit any significant RF emissions that might adversely affect image acquisition operation from within the MRI scan room and must not interact with the magnetic fields therein either to cause distortion of the field or to be influenced by these fields sufficiently to jeopardize reliable operation of such devices.